<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            position: relative;
            width: 444px;
            height: 320px;
            margin: 100px auto;
            background-color:pink;
        }
        .box img{
            width: 100%;
            height: 100%;
        }
        .box::after{
            content: '';
            display: none;
            position: absolute;
            left: 0;
            top: 0;
            width: 100%;
            height: 100%;
            background: rgba(0, 0, 0, .4) url(../images/arr.png) no-repeat center;
        }
        /* 鼠标经过盒子，就让遮罩层显示出来 */
        .box:hover::after{
            display: block;
        }

    </style>
</head>
<body>
    <div class="box">
        <img src="../images/tudou.jpg" alt="">
    </div>
</body>
</html>